Optimize LazyCell size

[joboet]

LazyCell can only store either the initializing function or the data it produces, so it does not need to reserve the space for both. Similar to #107329, but uses an enum instead of a union.

[rustbot]

r? @Mark-Simulacrum

(rustbot has picked a reviewer for you, use r? to override)

[rustbot]

Hey! It looks like you've submitted a new PR for the library teams!

If this PR contains changes to any rust-lang/rust public library APIs then please comment with @rustbot label +T-libs-api -T-libs to tag it appropriately. If this PR contains changes to any unstable APIs please edit the PR description to add a link to the relevant API Change Proposal or create one if you haven't already. If you're unsure where your change falls no worries, just leave it as is and the reviewer will take a look and make a decision to forward on if necessary.

Examples of T-libs-api changes:

• Stabilizing library features
• Introducing insta-stable changes such as new implementations of existing stable traits on existing stable types
• Introducing new or changing existing unstable library APIs (excluding permanently unstable features / features without a tracking issue)
• Changing public documentation in ways that create new stability guarantees
• Changing observable runtime behavior of library APIs

[workingjubilee]

I am not entirely sure if a #[cold] marker is correct here. This function is virtually guaranteed to be called (just... once).

[joboet]

Since OnceCell marks the closure as #[cold], I didn't want to change it just yet. But you are probably right that it is unnecessary here.

[Mark-Simulacrum]

r=me with some added comments.

[joboet]

@Mark-Simulacrum I added the comments, hopefully it is clearer now. There is also a (micro-) optimization in really_init that makes the code shorter, even if it might not help performance.

[bors]

☔ The latest upstream changes (presumably #105587) made this pull request unmergeable. Please resolve the merge conflicts.

[joboet]

Rebased to include the new feature name (lazy_cell) assigned in #105587.

[workingjubilee]

@bors r=Mark-Simulacrum

[bors]

📌 Commit af080bf has been approved by Mark-Simulacrum

It is now in the queue for this repository.

[bors]

⌛ Testing commit af080bf with merge 0599b6b...

[bors]

☀️ Test successful - checks-actions
Approved by: Mark-Simulacrum
Pushing 0599b6b to master...

[bors]

☀️ Test successful - checks-actions
Approved by: Mark-Simulacrum
Pushing 0599b6b to master...

[rust-timer]

Finished benchmarking commit (0599b6b): comparison URL.

Overall result: ❌ regressions - no action needed

@rustbot label: -perf-regression

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	1.4%	[1.4%, 1.4%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	2.1%	[2.1%, 2.1%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-2.5%	[-2.5%, -2.5%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.2%	[-2.5%, 2.1%]	2

Cycles

This benchmark run did not return any relevant results for this metric.